Analysis of mouse cDNA clones has led to the identification of a novel MHC class I (H-2)-related gene that encodes a truncated transplantation-like antigen. Unlike the products of the class I gene (designated H-2K, D and L) which are synthesized and displayed on the surface of all cells, the product of the class I-related gene is expressed only in liver cells and is secreted. We propose that this serum protein acts as a tolerogenic form of the transplantation antigen and is responsible for regulation of self-nonself recognition.